If two optical fibers are placed next to each other, will light jump from one to the other?
Under normal circumstances, two ordinary, intact optical fibers placed in close proximity will absolutely not allow light signals to “jump” (couple) from one to another.
From the perspective of optical physics and waveguide engineering, this involves several core technical principles:
1. Total Internal Reflection and Waveguide Confinement
Light can travel in an optical fiber because the fiber consists of a core ( n_{\text{core}} ) with a higher refractive index and a cladding ( n_{\text{cladding}} ) with a lower refractive index. Light undergoes total internal reflection at the interface between the core and cladding, remaining perfectly confined within the core and propagating forward without leaking outwards.
2. Evanescent Wave and Exponential Decay of Electromagnetic Field
Although light undergoes total internal reflection at the interface, from an electromagnetic perspective, the light field does not instantaneously become 0 at the core-cladding interface. A small portion of the light field penetrates into the cladding, which is known as the evanescent wave.
However, the electromagnetic field strength of the evanescent wave decays exponentially and rapidly within the cladding. The penetration depth of the evanescent wave is extremely shallow. For commonly used 1550\ \text{nm} communication infrared light, its penetration depth is typically only 1\ \mu\text{m} to 2\ \mu\text{m}.
3. “Physical Shielding” of Geometric Dimensions
Taking standard single-mode optical fibers as an example:
For these standard optical fibers, the core diameter is only 9\ \mu\text{m}, while the cladding outer diameter is 125\ \mu\text{m}, and the coating outer diameter is 255\ \mu\text{m}.
This means that the single-sided thickness of the physical barrier from the outer edge of the core to the outer edge of the cladding is approximately:
(125 - 9) / 2 = 58\ \mu\text{m}
Since 58\ \mu\text{m} is far greater than the 1\ \mu\text{m} to 2\ \mu\text{m} decay limit depth of the evanescent wave, by the time the light field reaches the outer surface of the cladding, its energy has already attenuated to a negligible absolute zero. Therefore, even if two optical fibers are placed in very close contact, the physical distance between their cores is at least over a hundred micrometers (even further if the coating is considered), making it impossible for any signal to transfer between fibers.
Under What Specific Circumstances Can Light “Jump” to Another Fiber?
In specific specialized device designs or engineering applications, through artificial intervention, the above physical limitations can be overcome, allowing light to couple across fibers:
-
Fused Biconical Tapering
If the coatings of two optical fibers are stripped, and they are fused and stretched at high temperatures until their claddings are extremely thin and the cores are brought within micrometers of each other. At this point, the evanescent fields of the two cores overlap, and light will couple from one fiber to the other. This is also the basic manufacturing principle for fiber splitters or optical couplers. -
Crosstalk in Multicore Fiber (MCF)
If multiple cores are fabricated simultaneously within a single glass cladding, and the spacing between the cores is designed to be too close, the evanescent waves will partially overlap, leading to “crosstalk”.
Specialized sensors utilizing this characteristic require extremely precise design during manufacturing. For example, OFSCN® Multicore Fiber Bragg Gratings / FBG Strings (Bare) are used in OFSCN® Fiber Bragg Grating Shape Sensors, which use precise core geometry distribution to avoid uncontrolled crosstalk while utilizing the tiny strain differences between multiple cores to accurately reconstruct the three-dimensional spatial form of the fiber. -
Light Leakage Under Extreme Bending (Bend Loss)
When an optical fiber is sharply bent (with a bending radius smaller than its allowable limit), some light leaks from the core into the cladding (transforming into cladding modes). Although this leaked light is dissipated, due to the obstruction of the outer cladding and coating of adjacent fibers, the leaked light is usually absorbed or scattered by the coating and still very difficult to couple into the core of the adjacent fiber.
Example Standard Optical Fiber Product Specifications
OFSCN® G.652D Optical Fiber
A standard G.652D single-mode optical fiber has a core diameter of 9\ \mu\text{m}, a cladding diameter of 125\ \mu\text{m}, and a coating diameter of 255\ \mu\text{m}.
OFSCN® G.657 Optical Fiber
A standard G.657 bend-insensitive single-mode optical fiber has a core diameter of 9\ \mu\text{m}, a cladding diameter of 125\ \mu\text{m}, and a coating diameter of 255\ \mu\text{m}.
